1. Field of the Invention
The present invention is related to an apparatus and methods for maintaining or recovering the performance and the functions of a magnetic recording head utilizing a giant magnetoresistive (GMR) effect in the manufacturing process in which the head is used in a hard disk drive (HDD).
2. Description of Related Art
The magnetic recording head using the GMR effect (hereinafter referred to as GMR head) provides a sensitivity three times or more of that of the magnetic recording head using the ordinary magnetoresistance (MR) effect (AMR head hereinafter referred to as MR head). On the other hand, better sensitivity means more liability to damage by an external stimulus than the conventional MR head. For instance, GMR head is susceptible to the effect of a stimulus such as electrostatic discharge (hereinafter, referred to as ESD). The ESD means that, when two materials are put in contact with or separated from each other through abrasion, pressurization, or collision, static electricity is generated on both materials and remains on an insulating material or insulated conductor (charging), and the residual static electricity is instantly discharged when it is put in contact with a material. In addition, stimuli such as electrical overstress (hereinafter referred to as EOS) and thermal stress (or temperature stress) also have undesirable effects on the performance of GMR heads.
Accordingly, in the manufacturing process in which GMR heads are finally assembled into an HDD, maintaining the head performance by decreasing the possibility of undergoing such damages and recovering the function of GMR heads having experienced such damages become important.
Through the diagrammatical view of FIG. 1, each process level is defined. The "wafer level" (a) is a stage at which GMR elements (GMR heads) are formed into a multilayered structure at the stage of a wafer 10. The multilayered structure is described later in FIG. 2. The "row level" (b) is the stage at which GMR elements are taken out from the wafer into a strip 12 to form a row. In these levels (a) and (b), GMR heads may get exposed to an external stimulus through a step such as cutting, washing, lapping, or polishing. Although, in the FIG. 1(a) a diagrammatical expression is given so that one rectangle corresponds to one of the GMR elements, it is possible to actually extract many more GMR elements than those shown in this diagrammatical view. The "HGA level" (c) is a head gimbal assembly (HGA) level, and more particularly a stage at which a GMR element is attached to the end face of a slider 14, which is mounted on a suspension assembly 16 for supporting the slider 14. In fact, the size of the GMR element is much smaller as compared with the size of the slider (the end face of the slider). The "HSA level" (d) is a level at which a plurality of HGAs are collected into a stack 18, and for a rotary actuator, they form an assembly rotatable on a pivot in unison. The "file level" (e) is a stage at which they are assembled into an HDD 20 and data writing and reading are allowed.
A class of GMR elements is known as spin valve (SV) magnetoresistance sensors as explained below. The SV magnetoresistance sensor is a sensor in which the electrical resistance between two uncoupled ferromagnetic layers varies as cosine of the angle between the magnetized directions of the two layers, independently of the direction of current. Thus, the SV sensor is different from the anisotropic magnetoresistance (AMR) in which electrical resistance varies as cos.sup.2 (square of cosine) of the angle between the magnetized direction and the direction of current.
FIG. 2 is a perspective view showing the multilayered structure made up of the respective layers forming a spin valve (SV) sensor 30. An underlayer or a buffer layer 33 is disposed on a substrate 31 as needed, and subsequently, a first thin-film layer 35 formed of a soft ferromagnetic material provided as a free layer is disposed on the buffer layer 33. In the presence of an external magnetic field, the magnetization of the free layer 35 can freely rotate (the dotted arrows). If no external magnetic field exists, the magnetized direction matches the direction of the solid arrow 32. A thin-film nonmagnetic metallic spacer layer 37 is subsequently disposed on the free layer 35 followed by disposing a second thin-film layer 39 of ferromagnetic material (pinned layer) on the spacer layer 37 followed by disposing a thin-film layer 41 made of anti-ferromagnetic material having a relatively high electrical resistance on the pinned layer 39. There is an exchange coupling interaction between the pinned layer 39 and the anti-ferromagnetic layer 41.
Ferromagnetic end regions 42 and 43 are formed adjacent to the end portions of the free layer 35 on the substrate 31. In the present invention thin layers of permanent magnet are provided as substitutes for the ferromagnetic layers 42 and 43. Layers 62 and 63 formed over layers 42 and 43, respectively, are electrical leads used for allowing current to flow in the SV sensor 30. The current for resetting the GMR head, which is described later, flows through the GMR head from the layer 62 to the layer 63 or from layer 63 to layer 62. That is, in the direction of applying a current or a voltage pulse to the GMR element. The direction in which the current flows is decided in the design, but the current flows in the direction in which the magnetic field generated by the current helps the magnetization of the pinned layer. This direction is the longitudinal direction of the GMR element represented by a rectangle in FIG. 1(a), the direction vertical to the longitudinal direction of the suspension assembly 16 in FIG. 1(c), and the direction in which a current i flows in FIG. 5.
Furthermore, it should be noted that the stacking sequence of the layers 35, 37, 39, and 41 may be reversed to a stacking sequence of 41, 39, 37, and 35. The combination of the layers 35, 37, and 39 is necessary for achieving the function of the SV sensor, but there is a choice as to the stacking sequence to provide better characteristics. In the present invention, the stacking sequence which is opposite to the stacking sequence shown in FIG. 2 is employed. The so-called air bearing surface of a magnetic recording disk is in the direction vertical to the direction of a signal field h. It is because the GMR element is attached to the surface corresponding to the end portion of the slider, as seen from FIG. 1(c) and FIG. 10.
The operation of orienting the polarity of the magnetized direction of the free layer by hard biasing by the use of an external magnetic field is called "initialization." Directly affected by the initialization are the thin layers 42 and 43 of permanent magnets where the magnetization of the end regions 42 and 43 is preferably biased in the longitudinal direction. The magnetized direction of the free layer is maintained by the layers 42 and 43. Note that the magnetization direction can be reversed by reversing the polarity of the external magnetic field (reversing the direction).
The GMR head needs to be magnetically and thermally stable, and in this respect, the stability of the exchange coupling between the ferromagnetic pinned layer 39 and the antiferromagnetic (AFM) thin-film layer 41 becomes a problem. In the manufacturing process in which the assembling is sequentially proceeded in the respective levels (a) to (e) of FIG. 1 described above, the exchange coupling may be weakened or completely disconnected by the damage caused by an external stimulus, so that the originally required function of the GMR head cannot be maintained.
To recover the function of the GMR head, an operation called reset is effective. The "reset" is to adjust the magnetized direction of the pinned layer.
The reason why the magnetized direction of the pinned layer can be adjusted is that, if the GMR head is heated to a temperature equal to or higher than the "blocking temperature" of the AFM layer by some action, the exchange coupling between the pinned layer 39 and the AFM layer 41 is once lost, and then the head is cooled in the presence of a magnetic field thereby pinning the pinned layer. Even in a state at which the temperature is lower than the blocking temperature, anisotropy tends to gradually decrease as the temperature increases.
It was found that the magnetized direction of the pinned layer may be adjusted by applying a simple relatively strong magnetic field. It is considered that this is because the magnetized direction partially disturbed in the magnetic domain may be adjusted. This is described later as a "high magnetic field method."
The reason why a damage is caused by a stimulus such as ESD or EOS is that portions heated to the blocking temperature or higher locally or globally appear in the layer of an anti-ferromagnetic material. However, in the reset operation, such phenomenon is artfully used.
However, there is a point to be noted. A heavy damage may be irreparable, and conversely, this means that there is a risk that, even an originally reparable light damage may be made destructive by a careless reset. However, effective reset of the GMR head requires application of a sufficiently high reset voltage.
FIG. 3 separately exemplifies a region A where the reset by a reset voltage V.sub.reset is not effective, a region B where the reset is effective, and a region C where a damage is caused. The reset voltage value is only an example. First, the reset must be carefully executed in the region B in which the reset is effective. Furthermore, the diagonally shaded portions are gray zones in which the respective regions A, B, and C cannot be clearly discriminated. Such variable regions are caused to appear by indefinite factors such as a difference among individual products. The area provided by removing the gray zones from the resettable region B is called a "resettable window." Accordingly, the region of the resettable window has a direct relation with the yield and reliability of magnetic recording heads utilizing the GMR effect.
FIG. 4 shows the general relationship between the reset voltage V.sub.reset of a GMR head and its output amplitude. The output amplitude has a (+) polarity and a (-) polarity. The phase is reversed by 180 degrees between the (+) and (-) polarities. There is a region in which the output amplitude varies little while the reset voltage is low, but, for instance, like a position 70, the output amplitude begins to increase when a predetermined reset voltage is exceeded. To make the output amplitude of the GMR head as high as possible, it is preferable that the reset is performed until the higher output amplitude is gained, if it is possible. On the other hand, like a position 80, a region appears in which the output amplitude does not increase as much regardless of the increase in the reset voltage after a predetermined reset voltage is exceeded. If the reset voltage is further increased, a damage is caused at and after a position 90 and then the output amplitude begins to decrease. Accordingly, if a desired output amplitude can be obtained, the reset voltage is preferably held at around the position 80 which is in a low state, if it is possible. The reason is that, if the reset voltage is increased to a position between 80 and 90, and even if a damage can be avoided, a damage is caused if the same reset voltage is reapplied later. This means that, if the reset is made between 80 and 90, damage is less likely than if reset were stopped at 80. The number of times the reset voltage is applied is also a factor of the possibility of damage, and thus the number of times is preferably as small as possible.
According to the above characteristics of the GMR element, the degree of damage or the performance degradation can be evaluated through the reduction of the output amplitude. By measuring the output amplitude, the degree of the degradation of the head function can be evaluated, and the degree of recovery can also be evaluated after the head function is recovered. One such evaluation method is called a "quasi-static test," in which generally the output amplitude is measured as the read back response of the GMR head in a low-frequency magnetic field. This quasi-static test may be called a magnetostatic test. That is, the GMR head is placed in an AC magnetic field (a low-frequency magnetic field the direction and intensity of which vary with time), and its read back response is measured. A bias current (sense current) is supplied to the GMR head and the read back signal is measured. In the GMR head, the output amplitude does not depend on the frequency of the magnetic field. This is one of the merits unique to the GMR element.
There are mainly three reasons for the occurrence of the degradation. The first reason is that the temperature in at least part of the GMR element becomes higher than the blocking temperature to release the pinning, as described above. This is a reversible change. The second and third reasons are generally that the film itself forming the GMR element is deteriorated. More specifically, the second reason is that an atomic movement or atomic diffusion is occurring in the film interface. The third reason is that the melting of the material forming the film is occurring. Among them, since the degradation by the first reason is reversible, it can be recovered by the reset. However, since those by the second and third reasons are irreversible, they cannot be recovered by the reset.
The degree of degradation can be evaluated mainly by the reduction of the output amplitude. However, a catastrophic degradation results in a permanent, irrecoverable reduction of output amplitude. In this respect, the evaluation is a problem if the GMR element has been damaged to a level which cannot be recovered by the reset because of the second and third reasons. As seen from FIG. 4, the output amplitude also appears as a reduction when the reset pulse passes the portion 90. If the degradation has reached such degree, the resistance value of the GMR head has often changed as compared with the previous resistance value. Thus, it is very useful that a determination is made as to whether the resistance value has changed.
In FIG. 5, various techniques (a) to (d) related to the reset of the GMR head are shown. Among them, the second technique (b) and the third technique (c) form the present invention by themselves, but they are also described in this section for ease of comparison with the first technique (a) and the fourth technique (d).
The first technique (a) is called a "thermo+magnetic field method." In this technique, an external magnetic field H.sub.ext is applied in a desired pinning direction in a temperature environment equal to or higher than the blocking temperature of exchange coupling followed by cooling down the GMR head. By this, the magnetization M is fixed. The heating can be performed, for instance, by a heater. The magnitude of the magnetic field is usually greater than 5000 Oe (oersted). This technique is ideal because the exchange coupling between the ferromagnetic layer and the anti-ferromagnetic layer can be uniformly obtained, but, if it is applied to a level of the manufacturing process after the wafer level, there is a side effect such as the deformation of other components (for instance, suspension) due to thermal expansion. Thus, it is not appropriate.
In the second technique (b) (the present invention), the GMR element is placed in a magnetic field, and a current or voltage pulse is applied to the element. The application of a pulse is intended to heat the GMR element to a temperature equal to or higher than the blocking temperature. Particularly in this case, a magnetic field equal to or greater than 5000 Oe is also applied in a desired pinning direction. The polarity of the pulse is preferably such that the magnetic field generated by the pulse is orienting in the direction of helping the pinning. But the reversed direction may be allowed if the external magnetic field is sufficiently high. This technique has a merit that the resettable window can be made relatively wide because of the help of the magnetic field, but it becomes complicated as a tool since a mechanism for generating a magnetic field must be provided in addition to the pulse. The above technique is called a "magnetic field+reset pulse method."
In the third technique (c) (present invention), a magnetic field H.sub.1 is applied to the GMR element in the desired pinning direction, and a magnetic field H.sub.2 is applied in the direction of initializing the hard bias. For instance, it is experimentally known that H.sub.1 is preferably 10 to 15 kG (G: gauss) or larger, and H.sub.2 is preferably about 5 kG. This technique has a merit that it has no risk of giving overstress and it has safety as long as they are magnetic fields of such intensity as can be commonly achieved. However, since consideration is made to safety, the effect is small without a high magnetic field, and there is little effect of adjusting the magnetized direction. This technique is called a "high magnetic field method." If safety is more emphasized, it is effective to perform the reset while gradually increasing the magnetic field which is an external stimulus.
In the fourth technique (d), by applying a pulse of a predetermined voltage (for instance, about one volt) to the GMR element for a predetermined short time (for instance, 100 ns), the magnetized direction of the pinned layer within the GMR element is adjusted to recover its function. In this technique, no external magnetic field is established, but the self-generation of a magnetic field by the pulse is utilized. Although it is a simple method, the heating to a temperature equal to or higher than the blocking temperature and the generation of the magnetic field for the pinning need to be combined as the pulse action, and thus the appropriate control is difficult. Accordingly, it has a defect that the resettable window is forced to be narrow as compared with the second technique. However, it has some degree of freedom as long as it satisfies the condition that the GMR element is heated to the blocking temperature or higher by properly selecting V.sub.R and T.sub.R with respect to the shape of the pulse, as written in the enlarged pulse in the second technique (b), and the condition that the magnetic field produced by a current i is oriented to the pinning direction of the pinned layer and sufficiently strong when the element is cooled below the blocking temperature during the decay of the pulse. For instance, it may stepwise decay, or may decay while exponentially decreasing. The above technique is called a "reset pulse method."
In any technique employed, it is important to efficiently execute the reset so that a sufficient effect is obtained without giving damage.